3
COMPARISON BETWEEN PREDICTED AND OBSERVED
COMPRESSIBILITY CHARACTERISTICS OF TREATED SOIL USING
PRECOMPRESSION AND VERTICAL DRAIN
AZHANI ZUKRI
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Engineering (Civil-Geotechnics)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
JUNE 2010
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To my beloved husband, sons and daughter..
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AKNOWLEDGEMENT
During preparing this thesis, I was in contact with many people, academicians
and practitioners. They have contributes towards my understanding and thoughts. I
wish to express my sincere appreciations to my supervisor, Associate Professor Dr
Kamarudin Ahmad, for encouragement, guidance, critics and motivation. I am also
thankful to my program coordinator, Associate Professor Dr Nurly Gofar and Ir
Azizan, Resident Engineer of UTHM Development Project, for their support and
advice. I would also to express my gratitude to Dr Ir Ramli Nazir for providing me a
data which is very important to my thesis. Without his help, the data for the project
would not be available for analysis. My appreciation also extends to all my
colleagues especially to Thanath, Vicky and Zalina who have provided assistance in
all occasions. Their views and opinion are useful indeed. Last but not least, I am very
grateful and indebted to my beloved husband Razali Kassim, my beloved mother Pn
Halijah Mahmood and my dad En Zukri Majid and all family members for their
support, patience and understanding.
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ABSTRACT
Prefabricated vertical drain, PVD with preloading is one of the widely used techniques to accelerate the consolidation process in ground treatment program. Sometimes, it is very difficult to choose proper design parameters during design stage. Therefore, back analysis from field records is a means to verify these design parameters. The purpose of this project is to evaluate the performance of ground improvement by pre-compression and vertical drain implemented for the construction of permanent campus of Universiti Tun Hussein Onn (UTHM), Parit Raja, Batu Pahat, Johor Darul Ta’zim by comparing the settlement design parameters between field and theoretical value. The anticipate magnitudes of settlements were calculated using Terzaghi’s one dimensional consolidation theory while the monitored settlements were evaluated using Asoaka’s method. The predictions of the total settlements with and without soil treatment had been made for three different zones. The time rate consolidation with the application of PVD was calculated using Hansbo’s solution adopting both vertical and radial consolidation effects. The back calculated coefficient of consolidation in horizontal direction, ch(back-cal) from Asoaka’s plot was performed using different time intervals and it was found that the longer time interval produced lower field coefficient of horizontal consolidation. The modified coefficient of horizontal consolidation, ch(modified) value also had been evaluated and it was concluded as ch(modified) = 0.22 ch (lab). This ch(modified) value would contribute to the formation of settlement curve resembling the actual settlement recorded from Asoaka’s plot. The ch(back-cal) value was about 1.04 to 3.59 m2/year. The value evaluated from dissipation test was found to be larger than ch(back-cal) value indicated that the smear effect was significant. However, this laboratory coefficient of horizontal consolidation, ch(lab) was in good agreement with normal assumption of ch = 2cv especially in Zone A while in Zone B and C the ratio was found to be 1 to 1.3. The ratio of ch(back-cal) over cv(lab) was found to be 0.27 to 0.96. The ratio of ch(modified)/ cv(lab) was in range 0.27 to 0.88 while the ratio of cv(field) / cv(lab) value was found to be 0.13 to 0.48. The correlation of coefficient of volume compressibility, mv(field) = 0.4 mv(lab) in Zones A and C, while the ratio was found to be 0.2 in Zone B. The compression ratio can be concluded as CR(field) = 0.03 CR(lab) and the compression index, Cc value can be summarized as Cc(field) = 0.03 Cc(lab) while the recompression index, Cr was found to be Cr (field) = 0.12 Cr (lab). This study shows that laboratory values were significantly higher than the field values.
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ABSTRAK
Saliran tegak pre-fabrikasi, PVD dan pra-bebanan surcaj merupakan salah satu teknik yang digunakan secara meluas untuk mempercepatkan proses pengukuhan tanah liat semasa rawatan tanah. Kadangkala sangat sukar untuk menentukan parameter-parameter yang sesuai semasa peringkat rekabentuk. Oleh itu analisis kembali daripada rekod ditapak digunakan bagi mengesahkan parameter-parameter rekabentuk yang digunakan ini. Tujuan kajian ini dilaksanakan adalah untuk menilai kemajuan tanah yang dibaiki dengan pra-bebanan dan saliran tegak yang dilaksanakan di kampus tetap Universiti Tun Hussein Onn (UTHM), Parit Raja, Batu Pahat, Johor Darul Takzim dengan membandingkan antara parameter rekabentuk teori dan sebenar di tapak. Jumlah enapan ramalan dikira menggunakan teori pengukuhan satu dimensi Terzaghi, manakala jumlah enapan sebenar di tapak dianalisa berdasarkan kaedah Asoaka. Penyelesaian Hansbo digunakan bagi meramal kadar masa pengukuhan bilamana PVD digunakan dengan megambil kira kesan pengukuhan menegak dan mendatar. Analisis kembali nisbah pekali pengukuhan dalam arah mendatar, ch(back-cal) daripada graf Asoaka di analisa menggunakan jangkamasa yang berbeza dan didapati analisis menggunakan jangkamasa yang panjang akan menghasilkan nilai pekali sebenar ditapak yang lebih rendah. Pekali pengukuhan arah mendatar diperbaiki, ch(modified) juga di analisa dan didapati ch(modified) = 0.22ch(lab). Pekali ch(modified) ini menyumbang kepada lengkung enapan ramalan yang menyamai lengkung enapan sebenar yang direkodkan oleh graf Asoaka. Nisbah pekali pengukuhan dalam arah mendatar, ch(back-cal) yang diperolehi adalah antara 1.04 hingga 3.59 m2/tahun. Nilai pekali yang diperolehi daripada ujian lesapan piezocone didapati lebih besar daripada nilai ch(back-cal) menunjukkan faktor kesan lumuran adalah penting. Walaubagaimana pun, nilai ch(lab) ini adalah bertepatan dengan kebiasaan anggapan ch = 2cv terutamanya di Zon A manakala di Zon B dan C pula nisbahnya adalah antara 1 hingga 1.3. Nisbah ch(back-cal) kepada cv(lab) didapati antara 0.27 hingga 0.96. Nisbah ch(modified)/cv(lab) adalah antara 0.27 ke 0.88 manakala nilai nisbah cv(field)/cv(lab) yang diperolehi adalah 0.13 hingga 0.48. Pekali kebolehmampatan isipadu adalah mv(field) = 0.4 mv(lab) di Zon A dan C, manakala di Zon B nisbahnya adalah 0.2. Nisbah mampatan yang diperolehi adalah CR(field) = 0.03 CR(lab) dan nilai indeks mampatan, Cc adalah Cc(field) = 0.03 Cc(lab) manakala indeks ketidakmampatan, Cr adalah Cr (field) = 0.12 Cr (lab). Kajian ini menunjukkan parameter yang diperolehi melalui ujian makmal adalah lebih tinggi daripada nilai sebenar yang diperolehi di tapak.
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TABLE OF CONTENTS
CHAPTER TITLE
PAGE
TITLE PAGE
DECLARATION
DEDICATION
ACKNOWLEDGEMENTS
ABSTRACT
ABSTRAK
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF SYMBOLS
LIST OF APENDICES
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ii
iii
iv
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xi
xiii
xvi
xix
1 INTRODUCTION
1.1 Background
1.2 Problem Statement
1.3 Objective of the Study
1.4 Scope and Limitation of the Study
1
3
4
4
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2 LITERATURE REVIEW
PAGE
2.1 Introduction 6
7
9
10
13
14
16
18
26
26
27
28
29
29
34
37
2.2 Soft Ground
2.3
2.4
2.5
Ground Improvement
2.3.1 Precompression or Preloading Technique
2.3.1.1 Types of Preloading
2.3.1.2 Preloading Method
2.3.2 Precompression and Vertical Drain
2.3.2.1 Design of Vertical Drain
2.3.3 Other Ground Improvement Method
2.3.3.1 Mechanical Stabilization
2.3.3.2 Chemical Stabilization
Consolidation
2.4.1 One-Dimensional Consolidation Theory
2.4.2 Compressibility Characteristics
2.4.3 Rate of Consolidation
Settlement
2.5.1 Immediate Settlement 37
39
42
44
44
2.5.2 Primary Consolidation Settlement
2.5.3 Secondary Compression Settlement or Creep
2.6 Settlement Prediction and Interpretation Method
2.6.1 The Oedometer Test
2.6.2 Cone Penetration Test (CPT) 47
2.6.3 Observational Method 48
2.6.3.1 Asoaka’s Method 48
2.6.3.2 Hyperbolic Method 51
2.7 Interpretation of Field and Laboratory Result for design
Purpose
53
2.8 Instrumentation and Monitoring Program 57
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2 LITERATURE REVIEW
2.9 Review of Previous Case Study
3 METHODOLOGY
3.1 Introduction
3.2 Data Acquisition
3.3 Data Analysis
4 CASE STUDY
4.1 Introduction
4.2 Subsurface Conditions
4.3 Soil Profile
4.4 Soil Characteristics
4.4.1 Zone A
4.4.2 Zone B
4.4.3 Zone C
4.5 Water Level
4.6 Soil Design Parameters
4.7 Construction Procedures
4.8 Instrumentation and Monitoring Program
4.9 Embankment Design
4.10 Acceptance Criteria
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5 ANALISYS AND RESULTS PAGE
5.1 Introduction
5.2 Magnitude of Settlement
5.2.1 Magnitude of Predicted Settlement without Soil
Treatment
5.2.2 Magnitude of Predicted Settlement with Soil
Treatment
5.3 Analysis of ch Value Using Asoaka’s Method
5.4 Back Calculated of Field Soil Characteristics
5.5 Correlation between Lab and Field Soil Characteristics
88
90
92
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100
101
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CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions
6.2 Recommendations
REFERENCES
105
107
108
APPENDIX 112
13
LIST OF TABLES
TABLE NO. TITLE PAGE
Table 2.1 Outline for identification of soft ground 8
Table 2.2 Range of Possible Field Values of the Ratio kh/kv for
Soft Clays (Rixner et al., 1986)
23
Table 2.3 Equations for the Compression Index from Different
Sources (adopted from Bowles, 1988)
31
Table 2.4 Compression and recompression indexes of natural soil 32
Table 2.5 Typical value of Poison’s ratio, v 38
Table 2.6 Typical Values of C (after Ladd, 1967) 43
Table 2.7 Modified Time Factor (after Gue & Tan, 2000) 55
Table 2.8 Instruments Suitable for Consolidation Monitoring 60
Table 2.9 Summary of Predicted Settlement 63
Table 2.10 Comparison of Soil Properties before and after Improvement 65
Table 2.11 Comparison between Recorded Settlement at Site and
Predicted From Laboratory Test Result (after Aw, 2001)
66
Table 2.12 Several Previous Findings from Different Researchers 66
Table 4.1 Generalized Soil Profile 76
Table 4.2 Soil Design Parameters at Zone A, B and C 79
Table 5.1 Expected Total Settlement Occurs After Construction 92
Table 5.2 Predicted Settlement and Time to achieve 90%
Consolidation
94
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TABLE NO. TITLE PAGE
Table 5.3 Final Settlement and ch Values 98
Table 5.4 Back Calculated Field Soil Characteristics 100
Table 5.5 (a) The correlation of coefficients of vertical consolidation,
cv
102
Table 5.5 (b) The correlation of coefficients of horizontal
consolidation, ch
102
Table 5.5 (c) The correlation between ch and cv 102
Table 5.5 (d) The correlation of coefficients of volume
compressibility, mv
103
Table 5.5 (e) The correlation of compression index, Cc 103
Table 5.5 (f) The correlation of recompression index, Cr 103
Table 5.5 (g) The correlation of compression ratio, CR 103
Table 5.6 Comparison of Settlement and Time to Achieve 90%
Consolidation
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LIST OF FIGURES
FIGURE NO. TITLE PAGE
Figure 2.1 Beneficial Effects of Preloading 12
Figure 2.2 Surcharge preloading is placed on top of a permanent
load (proposed embankment)
13
Figure 2.3 Staged Loading 14
Figure 2.4 Comparison of Dissipation of Excess Pore Water
Pressure for Projects with and without Installation of
PVD
17
Figure 2.5 Installation Process of Prefabricated Vertical Drain 17
Figure 2.6 Example of Comparisons between Time Curves
(Stamatopoulos & Kotzias, 1985)
18
Figure 2.7 Relationship of Drain Spacing (S) to Drain Influence
Zone (D) (after Rixner et al., 1986)
21
Figure 2.8 The Disturbed Zone around Mandrel (Rixner et al.,
1986)
24
Figure 2.9 Schematic of PVD with the resistance and soil
disturbance (Rixner et al., 1986)
25
Figure 2.10 Typical Shape of Vertical Drain 25
Figure 2.11 Plot of void ratio versus pressure in logarithmic scale 30
Figure 2.12 Consolidation curve for two-way vertical drainage
(Hausmann, 1990)
36
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FIGURE NO. TITLE PAGE
Figure 2.13 Illustration of settlement of footing on elastic medium 39
Figure 2.14 Possible loading conditions for a (a) normally
consolidated clay and an (b) over consolidated soil (after
Das, 2004)
42
Figure 2.15 Consolidation Settlement 45
Figure 2.16 Asoaka Diagram 49
Figure 2.17 Asoaka’s Graphical Method (after Tan and Gue, 2000) 50
Figure 2.18 Hyperbolic Method 51
Figure 2.19 Example of Hyperbolic Plot Method (Tan, 1996) 52
Figure 2.20 Example Defining Soil Layer and Interpreting Soil
Names (after Dobis & Wong, 1990)
54
Figure 2.21 Normalized Pore pressure Dissipation Versus T* 56
Figure 2.22 Typical Arrangement of Instrumentations for
Monitoring Program
59
Figure 2.23 The Diagram of Open Well and Standpipe Piezometer 61
Figure 2.24 The Schematic of Pneumatic Piezometer 62
Figure 2.25 The settlement point is used to monitor settlement of
soil under an embankment
62
Figure 2.26 The Principle of Inclinometer Operation 62
Figure 2.27 Settlement plot with and without vertical drain 64
Figure 3.1 Flowchart of the Research Methodology 68
Figure 4.1 The Topographical Map of Development Area 73
Figure 4.2 The Geological Map of Development Area 74
Figure 4.3 The Boreholes Location 76
Figure 4.4 Excavation of unsuitable material 80
Figure 4.5 Laying of Geotextile 81
Figure 4.6 Installation process for PVD 82
Figure 4.7 Installation of PVD 82
Figure 4.8 Cutting off vertical drain 83
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FIGURE NO. TITLE PAGE
Figure 4.9 Locations of Instrumentations 85
Figure 4.10 Locations of Ground Improvement Area 87
Figure 5.1(a) Detail of Embankment and Subsoil Model for Zone A 89
Figure 5.1(b) Detail of Embankment and Subsoil Model for Zone B 89
Figure 5.1(c) Detail of Embankment and Subsoil Model for Zone C 90
Figure 5.2 Percentage Consolidation versus Time at Zone A,
B & C
94
Figure 5.3 ch Value versus Time Interval 95
Figure 5.4 Comparison of ch Value from Different Method of
Analysis
96
Figure 5.5 (a) Predicted Settlement versus Time (Zone A) 97
Figure 5.5 (b) Predicted Settlement versus Time (Zone B) 97
Figure 5.5 (c) Predicted Settlement versus Time (Zone C) 97
Figure 5.6 (a) Time Rate Settlement (Zone A) 98
Figure 5.6 (b) Time Rate Settlement (Zone B) 99
Figure 5.6 (c) Time Rate Settlement (Zone C) 99
Figure 5.7 Comparison of mv Value from Different Method of
Analysis
101
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LIST OF SYMBOLS
a - Width of Strips
av - Coefficient of Compressibility
BH - Borehole
B - Width
B,t - Thickness of Strips
C - Compressibility coefficient
CPT - Cone Penetration Test
Ca - Modified of Secondary Compression Index
C - Rate of Secondary Compression
Cc - Compression Index
ch - Coefficient of consolidation in Horizontal Direction
CR - Compression Ratio
Cr - Recompression Index
Cu - Undrained Cohesion of Soil
cv - Coefficient of Consolidation in Vertical Direction
dw - Equivalent diameter of prefabricated vertical drain
E - Young Modulus
Eu - Undrained Modulus
e - Void ratio
e0 - Initial void ratio
e1 - Final void ratio
G - Shear Modulus
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Gs - Specific Gravity
H - Thickness of the Compressible Layer
Hd - Length of Drainage Path
Ip, - Influence Factor
Ip - Plasticity Index
Ir - Rigidity Index
k - Coefficient of Permeability
kh - Horizontal Coefficient of Permeability
ks - Permeability of Smeared Zone
kv - Vertical Coefficient of Permeability
LL - Liquid Limit
n - Ratio of D/d
mv - Coefficient of Volume Compressibility
OCR - Over-Consolidated Ratio
p - Pressure
PI - Plastic Index
PVD - Prefabricated Vertical Drains
q - Extra surcharge
qc - Cone Resistant
qu - Unconfined compression Strength
r - Radius
RR - Recompression Ratio
s - Drain Spacing
Sc - Primary Consolidation Settlement
Si - Settlement Reading, Immediate Settlement
Si-1 - Preceding Settlement
Sf - Final Settlement
Ss - Secondary Compression Settlement or Creep
SPT-N - Value of Standard Penetration Test
su - Undrained shear strength
t - Time
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t - Time of consolidation
tf - Time of Interest for Secondary Consolidation
tp - Time of The Beginning of Secondary Consolidation
Th - Time factor for Horizontal Drainage
Tr - Time factor for Radial Drainage
Tv - Time Factor
UTHM - Universiti Teknologi Tun Hussein Onn Malaysia
U - Degree of Consolidation
Ue - Excess Pore Water Pressure
Ur - Average Degree of Consolidation Ratio for Radial Drainage
U v - Average Degree of Consolidation Ratio for Vertical Drainage
Uvr - Average Degree of Consolidation Ratio for Combined Vertical and
Radial Drainage
v - Poisson’s Ratio
WL - Liquid Limit
Wn - Natural Water Content
z - Depth
α - Constant Which Depends on The Spacing Ratio of The Vertical Drain
Δe - Change in Void Ratio
Δp - Change in Stress
Δσ’ - Change in Effective Stress
σ'0 - Initial Effective Stress or Existing Overburden Pressure
σc' - Pre-consolidation Pressure
w - Water Unit Weight
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LIST OF APPENDICES
APPENDIX NO. TITLE PAGE
Appendix A Prefabricated Vertical Drain Specification 112
Appendix B Soil Characteristics at Zone A 113
Appendix C Soil Characteristics at Zone B 116
Appendix D Soil Characteristics at Zone C 119
Appendix E Predicted settlement calculation Using One
Dimensional Terzaghi's Analysis for Zone A, B and C
123
Appendix F Calculation of Embankment Settlement 124
Appendix G Consolidation Analysis Using Hansbo’s Solution for
Zone A, B and C
127 Appendix G2 Sample Calculation Consolidation Analysis Using
Hansbo’s Solution
130
Appendix H Asoaka's Analysis for Zone A, B and C 133
Appendix I Sample Calculation For Asoaka’s Plot 135
Appendix J Back calculated of Field Soil Characteristics 137
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CHAPTER 1
INRODUCTION
1.1 Background
Soft ground deposits are widespread and cover many coastal regions of the
world, such as Japan, Eastern Canada, Norway, Sweden and other Scandinavian
countries, India and the South East Asian countries. In Malaysia, soft soil deposits
are widespread all over the country and mostly found in the coastal area. Further
extensive development activities are apace in many lowland areas to promote human
activities such as agriculture, industry, housing, infrastructure facilities and so on.
With the decrease in suitable land for construction, the next choice available is to
expand the development in the geotechnical challenging environment such as
wetlands or soft soil area and highlands.
2
Generally, soft soils exhibit low strength and high compressibility. Many are
sensitive, in the sense that their strength is reduced by mechanical disturbance. For
example, foundation failure is likely in structure constructed in soft soil and surface
loading beyond yield stress levels due to embankments and shallow foundation
inevitably results in large settlements. This is because, these soils respond in a
spectacular manner to stress changes, that geotechnical engineer has an obligation to
examine to what extend the soft soils can be analyzed within the frame work of
development in soil mechanics. The soft soils conditions need to be improved to
avoid the excessive settlement and to ensure the stability and safety of the built
infrastructure and other facilities.
Nowadays, many ground improvement methods have been proposed and
implemented on soft soils to improve the bearing capacity and minimize the
settlement. In geotechnical engineering, ground improvement means the increase on
soil shear strength, the reduction of soil compressibility and the reduction of soil
permeability 1. Precompression by preloading is one method to improve the bearing
capacity and reduce the settlement. It is most effective and economical compare to
other methods such as mechanical and chemical stabilization, soil reinforcement such
as using granular piles and stone column.
Preloading is usually combined with vertical drain to speed up the
consolidation process, hence, reduce the post construction settlement. Prefabricated
vertical drain (PVD), or sometime referred to band or wick drains have been widely
used in Malaysia as a means to accelerate the consolidation settlement.
3
1.2 Problem statement
Generally, the site investigation is performed to obtain information of the
physical properties of soil within a site to design earthwork and foundations for the
proposed structures. Such investigation will include surface and subsurface
exploration to enable soil sampling and laboratory tests of the soil samples retrieved.
The test on some soil design parameters are not normally being carried out
during SI stage, unless special provisions are made. Some design parameters are
usually being assumed based on published literature, report and experiences during
the designing stage in the absence of adequate data. Inaccurate assumptions would
result in problems that the actual settlement differs from the predicted or designed
values.
The precompression or preloading and prefabricated vertical drains (PVD)
have been used successfully as a ground improvement method for soft soil. The
performance of this ground improvement method can be evaluated by comparing the
design parameters of the subsoil before and after stabilization work.
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1.3 Objectives of the study
The aim of this study is to see the difference between predicted or designed
parameters from actual condition on site through the analysis of the data that being
collected from instrumentations and monitoring program. In order to achieve the aim
of the project, the study consists of the following objectives;
i) To predict the magnitude of settlement by adopting the Terzaghi’s
consolidation theory and time required based on Hansbo’s solution for
PVD consolidation.
ii) To determine field coefficient horizontal consolidation, ch value based
on settlement data by adopting Asoaka’s method.
iii) To determine the correlation of compressibility characteristics by
comparing the magnitudes of predicted settlement and monitoring
settlement.
1.4 Scope and limitation of the study
This is a case study based on a development project in Parit Raja, Batu Pahat,
Johor Darul Takzim. The subsoil condition, loading type, method of construction,
monitoring and instrumentation system, soil improvement method and PVD are
being discussed in this research. The scope of the analysis will be limited to
settlement criteria.
5
Analysis would be based on the available limited Site Investigation and
instrumentations monitoring records. Series of instrumentation program such as
settlement plates, pieozometers, inclinometers and extensonometers were installed to
monitor the performance of the treated ground. 24 locations of Settlement plate
instruments was installed to monitor the settlement of the embankment in whole
development site, however, only 9 sets of settlement data in Zone A, B and C will be
used for analysis in this particular study. Calculation being carried out using manual
calculation and for more complicated analysis, Microsoft Excel spreadsheet program
will be used.
The back analysis of the settlement data from the field data would be carried
out by adopting the Asoaka’s method. The effect of vertical drain in speeding up the
consolidation process was evaluated based on the both effects of horizontal and
vertical conditions. However, some consolidation parameters and data were assumed
to be fixed for calculation purpose due to the inter-dependency between the
parameters used, which were initial void ratio, e0, permeability of smeared zone
coefficient, ks, horizontal coefficient of permeability, kh and vertical coefficient of
permeability, kv. The ratio of kh/ks was taken as 2 as recommended by Zhou et al 1.
Type of prefabricated vertical drain, PVD used for this project is Nylex
Flodrain and the PVD specification as attached in Appendix A. The PVD was
installed in close spacing with distance 1.5 meter for Zone B and 1.0 meter for Zone
A and Zone C in triangular configuration and 16 meters in length for each point.
